Newborn bison calves have joined the herd at Midewin National Tallgrass Prairie in Wilmington.

The herd of 27 — four bulls and 23 cows — were first introduced to the prairie in October 2015, and have grown to about 50, with more babies expected to be born through June, said Veronica Hinke, Midewin spokeswoman.

Since the beasts freely roam the range, there is no way to speculate on exactly how many bison are here, she said.

They were brought to Midewin as part of a 20-year experiment to see if their grazing patterns would help restore the 19,000 acre prairie, the former site of the Joliet Army Ammunition Plant.

The Iron Bridge Trailhead on Route 53, north of Midewin's Welcome Center is the main access point to the bison range.

Visitors can hike on their own or join one of the free guided hikes with rangers, which are conducted at 11 a.m. every Saturday and Sunday through Nov. 4, from the Iron Bridge Trailhead.

The ranger trailer will be open from 10 a.m. to 3 p.m. to provide information to visitors about the bison herd and life on the restored prairie.

The hike can go for as little as a half mile, or up to three miles, as guides lead hikers to where the herd was last spotted. There are no guarantees of seeing the animals, since their grazing patterns are unpredictable. Visitors should bring binoculars.

The Welcome Center, 30239 IL Route 53, is open from 8 a.m. to 4:30 p.m., Monday through Saturday from now through Nov. 4. It offers trail maps, archaeology exhibits, and telescopes to spot bald eagles, as well as books and gifts. Staff and volunteers are available to answer questions.